The present invention relates to a method and an apparatus for the thermal treatment of objects in a heating unit, especially semiconductor wafers in a rapid heating unit, according to which the objects are thermally treated with a prescribed temperature course, and the temperature of the object is regulated with an appropriate temperature regulation, e.g. a PID regulation and a forward-acting control that is based upon a simulation model of heating apparatus and object.
Such methods and apparatus are known in the art. For example, in the semiconductor industry for the manufacture of electronic components, it is customary to thermally treat disk-shaped semiconductor substrates via heat lamps having high heating rates of more than 100xc2x0 C. per second. In this connection, the thermal treatment generally follows a prescribed chronological temperature profile. To achieve this temperature profile, a regulation of the heating power emitted from the lamps is necessary. Since the heating lamps are controlled with a prescribed power profile, the wafer temperature follows a specific temperature curve. However, in this connection one must take care that between the radiated power given off from the lamps and the temperature of the wafer there is no linear relationship, which is attributable to different effects, in particular the Stefan-Boltzmann principle, (as described, for example, in U.S. Pat. No. 4,761,538), but also, for example, the shape of a process chamber, the arrangement of various elements within the process chamber, the position of the wafer relative to the heating lamps, etc. Therefore, a simple control of the temperature profile via a prescribed control of the lamps is not possible.
For this reason, there is effected a constant monitoring of the wafer temperature at any given time along with simultaneous readjustment thereof if there is a deviation from a theoretical temperature value. In this connection, two different regulating processes are utilized, namely a closed temperature regulating circuit, e.g. a PID regulation on the one hand, and a so-called forward-acting control on the other hand.
In the following, one should speak of regulation if at least one parameter of a system should be brought to a value (or within an interval about this value), whereby this parameter is conveyed back to a regulating apparatus so that the regulating apparatus can adjust the desired value as optimally as possible as a function of the observed parameter of the system. In this connection, the parameter can be detected directly in the system, for example by measurement, although it can also result, for example, from a model that reproduces the system in as good a manner as possible. Here one speaks of model-based regulation. Similarly, with systems that are regulated with regard to several parameters, a combination of model-based and first-mentioned return of the parameters can be present. In general, one designates the return of such parameters as feedback coupling.
In contrast to the regulation, with the control the parameters of the system that are to be controlled are not returned to a control device. The parameters that are to be controlled are determined with the control device, e.g. by a model, and/or are controlled via some other parameter than the parameter that is to be controlled.
With a closed temperature regulating circuit, the actual value of the wafer temperature at any given time is compared with a prescribed theoretical or desired value. If deviations occur between the two values, a regulating apparatus becomes effective and takes care of an adjustment of the two values by more or less controlling, for example, the heating lamps. The greater the regulating difference is, the greater is the readjustment. Drawbacks of this regulation are a) that the regulating device is not informed about future changes of the theoretical value, and b) that the wafer characteristics are not taken into consideration, which can vary during the regulating process, for which reason such a regulation cannot react in an anticipatory manner.
These drawbacks are compensated for by a forward-acting control that in addition to a previous development, namely the theoretical value and the actual value at any given time, also draws in the future development of the theoretical value into the regulating process. As a consequence, the adaptation of the actual value to the theoretical value becomes more precise, since the regulating apparatus draws in future changes of the theoretical value into the regulation.
For an even more precise regulation, a future property of the theoretical value is calculated in advance, and in particular with the aid of a simulation model comprising heating apparatus and object or wafer that is to be treated. In this case, one speaks of forecast-regulated processes. Since the thermal capacities of individual chamber components are known, and it is known which lamp power is radiated into the chamber, the wafer temperature, as well as its future development, can be estimated in advance by the forecast of the simulation model as a function of the progress of the profile of the heating power.
This estimation, which up to now was effected upon a rigid simulation model, is, however, very difficult, since the different components in the process chamber, including the heating apparatus and the object that is to be treated, represent a non-linear system. Despite these difficulties, with this method the adaptation of the course of the wafer temperature to the threshold profile can be improved.
As already mentioned, such simulation models treat the chamber with all of its individual components and the wafer together as one system. There is no distinction between individual system components. Furthermore, the previously known simulation models are established one time and are subsequently not altered, especially not during a process, i.e. while the object experiences a temperature-time treatment. Alterations within the system, for example during the treatment of different wafers (objects) having different optical characteristics, cannot be taken into account. In particular, alterations caused by process progress and/or by aging, such as, for example, the radiation given off by a heating lamp or other alterations within the chamber, cannot be taken into account. Changes caused by the progress of the process are, for example, heating up of the process chamber, which is made, for example, of quartz glass, and the thermal radiation that additionally results therefrom and that is in a wavelength spectrum that in general differs from that of the lamp radiation.
Proceeding from this state of the art, it is therefore an object of the present invention to provide a method and an apparatus for the thermal treatment of objects in a heating unit that enables a better regulation of a temperature profile of an object that is to be treated.
Pursuant to the present invention, this object is realized in that the simulation model includes at least one individual model that includes components of the heating apparatus and/or of the object, and in that at least one parameter of at least one of the individual models is monitored during the thermal treatment and in that the simulation model is adapted to at least one of the monitored parameters. This results in the advantage that the simulation model can be dynamically adapted to varying operating conditions, such as, for example, alteration of the heating power of the lamp due to age, objects having different optical characteristics, etc. Due to the adaptation of the simulation model, a more precise regulation of the temperature curve of the object that is to be treated is in particular also possible for the reason that advantageously alterations that are due to the progress of the process, such as, for example, the aforementioned heating up of, for example, process chamber (especially also the quartz components contained therein) can also be taken into consideration during the temperature regulation. This can be utilized advantageously, for example, for the reduction of the so-called xe2x80x9cfirst waferxe2x80x9d effect. This involves the influence of the process chamber temperature upon the process result during the processing of wafers if, for example, during the processing of the first wafer the process chamber has not yet reached its average xe2x80x9coperating temperaturexe2x80x9d. This effect always occurs at the beginning of, for example, a mass production, or if between the processing of individual wafers there is so much time that the process chamber can cool off to temperatures that are below that of, for example, mass production. As a result, due to the equipment, the process results can be a function of the throughput of the wafers, which of course is not desired. Pursuant to one preferred embodiment of the invention, the object is irradiated with at least one heating lamp of a heating device. An individual model having at least one monitored parameter is preferably provided for at least one heating lamp of the heating device, and operating parameters of the heating lamp, in particular the irradiated heating power in relation to the control power, are monitored in order to discover alterations and if necessary adapt the simulation model.
Pursuant to a further preferred embodiment of the invention, an individual model is provided for the object that is to be treated, and parameters of the object that is to be treated, especially optical characteristics thereof, are monitored in order to undertake, if necessary, an adaptation of the simulation model. Of particular significance are the absorption characteristics (or in general the optical characteristics such as transmission, absorption or reflection) of the object that is to be treated or the coupling to the heat radiation at different temperatures, which can greatly influence the regulation, especially a forward-acting control, since these characteristics are greatly temperature dependent for, for example, Si wafers. The parameters are preferably separately determined from one another on opposite sides of the object.
For a further optimization of the overall model, the data transmission times and/or the computing times are determined and individual models provided herefor are adapted to the determined values. With some measuring devices, such as, for example, pyrometers, a temperature determination of the object is not possible, or is only possible with great difficulty, below 400xc2x0 C. Therefore, the temperatures of the object under 400xc2x0 C. are preferably calculated at a later stage with the aid of the simulation model, and this calculated information is taken up in the regulation.
The object of the invention is also realized with an apparatus for the thermal treatment of objects, especially semiconductor wafers, with a heating device, especially a rapid heating device, a regulating unit having a temperature regulator, and a forward-acting control that utilizes a simulation model of heating apparatus and object, in that a monitoring unit is provided for the sensing of parameters of components of the heating device and/or of the object, which parameters are relevant for the simulation model, for the comparison of the measured parameters with the parameters of the simulation model and for the adaptation of the parameters of the simulation model to the measured parameters. With this apparatus there result the advantages already mentioned above with reference to the method.